The present invention relates to a liquid crystal electro-optical device utilizing chiral smectic liquid crystals.
Liquid crystals are utilized in many kinds of electro-optical devices such as display devices, shutter arrays for printers and shutters for cameras, because liquid crystals have many excellent characteristics such as being small and thin, and their electric consumption being low.
Recently, liquid crystal panels utilizing ferro-electric liquid crystals such as those of chiral smectic C phase (hereinafter stated as Sm C*) have received attention as electro-optical devices having such effects as high-speed response ability and memorizing characteristic.
One of the well known liquid crystals which has this chiral smectic C phase, is, for example, 2-methyl butyl P-[(P-n-decyloxy benzylidene)amino]. As shown in FIG. 3, this liquid crystal is oriented in one direction, with a spiral structure twisted in each layer unit.
When this Sm C* is poured into a gap between two plates B, the gap being about 1 .mu.m, smaller than the spiral period (usually several .mu.m) (see FIG. 4a), the liquid crystal molecule loses its spiral structure, the molecule axis becomes parallel to the plate, and is oriented so that the molecules are inclined .theta. from the normal direction of the layers.
Therefore, the liquid crystal poured into the cell has two conditions in two domains, one domain which is inclined +.theta. degrees in a clock-wise direction from the normal line of the layer, and the other domain which is inclined -.theta. degrees in a counter-clock-wise direction, as shown in FIG. 5.
A Sm C* liquid crystal molecule has an electric dipole vertical to the molecule axis, and when in one domain the electric dipole points upwardly to the cell plate, in the other domain the electric dipole points downwardly. Therefore, when an electric field is applied between the cell plates, the liquid crystal molecules inside the cell are oriented either +.theta. or -.theta. inclined from the normal direction of the layer, and when the applied electric field is reversed, the molecule is reversed and becomes oriented at a position either +.theta. or -.theta. inclined. It is needless to say that, because the liquid crystal molecules have polarizing characteristics, when polarizers are mounted on both faces of the above cell, an optical bright-dark condition occurs by reason of the orientation direction of the liquid crystal molecules, and so it is possible for them to function as liquid crystal display panels and as shutter arrays.
A liquid crystals panel utilizing Sm C* liquid crystal as described above, has important features such as a very high response speed at the micro second level, and the ability to hold the pattern for a long period even when the electric field is removed after the display.
The fact that chiral smectic liquid crystals have these characteristics, was first announced by Clark and Lagerwall (Appl, Phys. lett. 36, 899, 1980).
Also they claimed that chiral smectic liquid crystals have another characteristic, i.e. "the desirable threshold value characteristic". The threshold value characteristic in this case, is not the threshold value characteristic towards the effective voltage in a TN type liquid crystal, but is the characteristic only towards the applied voltage value.
But, there are no data to show the existence of a desirable threshold value in our experiments or in any other research institution.
Therefore, we developed a driving method which selects the on-off conditions by the selecting voltage .+-.Vap having a desirable pulse width at selecting time, and which memorizes the on-off conditions with the AC pulse smaller than the said Vap, the AC pulse having equal positive-negative amplitude as described in Japanese Application No. 232963/83.
FIGS. 6(a), (b) show embodiments of drive wave-form used in the above driving method.
FIGS. 6(a), (b) show respectively the drive wave-forms in which the electric potential relative to the scanning electrodes are +Vap, and -Vap, at the selecting time.
When chiral smectic liquid crystals are driven by said drive wave-form, conditions such as whether the display is good or bad depend greatly on the orientation of the liquid crystals.
The reason is, that the molecule orientation by a uni-axial alignment process is stable, but the situation when voltage is applied and the molecules are parallel to the plate, leads to severe deformation.
FIG. 7 shows the molecule orientation, when a uni-axial alignment process has been carried out on the surface of the two plates.
The layers are thought to be inclined toward the thicker part of the plate, and when referring to one molecule, and when assuming an imaginary cone as in FIG. 3, the molecule is thought to be situated at the top surface of the cone as shown in FIG. 8. The molecules are thought to be parallel to the plate, and situated at the top surfaces of the imaginary cones.
It is necessary to position the molecules parallel to the plate, and to stably position the molecules at one of the two bi-stable molecule positions on the center surface of the imaginary cone.
Conventionally, to obtain this condition, the method claimed by Professor Fukuda of Tokyo Institute of Technology, is known, where the molecule is oriented from the side with a spacer and the like, and then takes the position parallel to the plate on the center surface of the imaginary cone.
But with this method, the spaces holding the side orientation force must be placed in the cell in large numbers, and it is difficult to utilize mass production on an industrial scale.